Exploring the geological features and processes that control the shape and internal fabrics of late diagenetic dolomite bodies (Lower Khuff equivalent – Central Oman Mountains)

Abstract

The Lower Permian carbonate rock succession of the Oman Mountains represents a stratigraphical surface analogue for Middle Eastern Lower Khuff hydrocarbon reservoirs. The characteristics of the carbonate rocks are highly variable both laterally and vertically due to the superposition of several episodes of dolomitization. A stratabound early dolomitization event affected the well-bedded limestones during Triassic times. Subsequently, a later burial dolomitization led to the emplacement of several hundreds of meter wide and tens of meter thick dolomite bodies. The goal of our study is to understand the distribution and genetic controls on the formation of late diagenetic dolomite bodies.Field mapping in Wordian to Capitanian beds was conducted in the Jebal Akhdar tectonic window in the Central Oman Mountains to determine the shape, internal fabrics and associated structures of late diagenetic dolomite bodies. The longest extension of late diagenetic dolomite bodies was found to follow predominantly the strike direction of fracture planes, bed-to-bed contacts, the Precambrian–Permian unconformity and the bottom rim of early diagenetic dolomite. Two groups of diagenetic fabrics were observed in association to fractures and bedding planes. First, vug related fabrics comprise vuggy pores and zebra dolomite. Second, breccia related fabrics consist of either small-scale crackle/mosaic breccia fabrics and centimeter sized vugs followed by bedding-parallel vugs lined up. Petrographic analysis revealed three types of diagenetic phases referred to as D1, D2 and D3. The matrix replacive phase D1 represents the main diagenetic phase formed during burial. Coarse crystalline dolomite rhombs (phase D2) and saddle dolomite (phase D3) were found along stylolites, in vugs and in fractures. Based on the presence of saddle dolomite and zebra dolomite textures, late diagenetic dolomite bodies are interpreted to have formed by the circulation of warm dolomitizing fluids of at least 60 °C.